1. Field of the Invention
The present invention relates to a cathode ray tube (CRT), and more particularly, to a CRT having reduced convergence drift, including a metal coating layer electrically connected to a built-in graphite layer on the inner surface of a neck portion.
2. Description of the Related Art
In general, if power is applied to a CRT, an electron gun emits electron beams from a cathode. The emitted electron beams pass through electron beam apertures of a plurality of electrodes are focused and accelerated. The accelerated electron beams are selectively deflected by a deflection yoke installed in the cone portion of a bulb and excite a phosphor layer coated on the inner surface of a panel which forms a screen, thereby producing a picture.
As shown in FIG. 1, a conventional CRT 10 includes a panel having a phosphor layer on its inner surface, a funnel 12 sealed in the panel 11, and a shadow mask 13 inwardly spaced from the panel 11.
The shadow mask 13 is coupled to a shadow mask frame 14. The shadow mask frame 14 is fixedly positioned to a stud pin 15 on the inner surface of the panel 11 and a hook spring 16 connected to the stud pin 15. Accordingly, the position of the shadow mask 13 in the panel 11 is determined.
An electron gun 17 for generating electron beams producing red (R), green (G) and blue (B) light, is inserted into a neck portion 12a of the funnel 12. A deflection yoke 18 for deflecting the electron beams, is installed in a cone portion 12b of the funnel 12.
An inner graphite layer 19 and an outer graphite layer 100 coated on inner and outer surfaces of the funnel 12, respectively, and thus a high voltage applied to an anode can be stabilized by forming a condenser using the glass funnel 12 as an insulator.
As known very well, the electron gun 17 includes a triode consisting of a cathode, a control electrode and a screen electrode, a plurality of focusing electrodes opposed to the screen electrode, for forming a pre-focusing lens unit, and a final accelerating electrode opposed to the focusing electrodes, for forming a main focusing lens unit.
A shield cup 110 is installed in front of the electron gun 17. A plurality of bulb spacers 120 are fixed on the outer circumference of the shield cup 110. The bulb spacers 120 elastically contact the inner graphite layer 19 to supply a positive voltage to the final accelerating electrode.
The CRT 10 must optimize the convergence characteristic by which R, G and B electron beams emitted from the electron gun 17 converge onto a point throughout a screen, inclusive of the center and corners of the screen. In the CRT 10, when the electron beams are deflected, they may be shifted from their proper positions, a phenomenon which is called convergence drift.
The convergence drift is divided into thermal drift and charge drift. Specifically, the charge drift is caused by a change in the potential of the neck portion 12a due to the condition of the outer surface of the neck portion 12a when a high voltage is applied to the CRT 10. The initial potential of the neck portion 12a is attributed to accumulation of charge due to electron beam current, causing an increase in the convergence error.
To overcome the problem, U.S. Pat. No. 4,868,454 discloses a method of stabilizing the potential of the surface of the neck portion with a metallic mirror coating on the inner surface of the neck portion. However, according to this method, convergence drift is 0.2 mm or greater, that is, the effect of removing charge is weak and occurrence of arcing is highly probable.
U.S. Pat. No. 5,536,997 discloses that an enamel layer electrically contacting a conductive layer coating the inner surface of a neck portion. The formation of the enamel layer relatively reduces convergence drift. However, this method has the following problems. First, the process of manufacturing a CRT is relatively complex. In other words, a conductive layer made of graphite is applied to the inner surface of the neck portion and dried. Then, an enamel glass solution is placed in contact with the conductive layer. During this procedure, the conductive layer and the enamel layer are electrically connected. Thus, forming the enamel layer is further necessary. Second, the arcing characteristic is poor. In the course of sealing the electron gun into the neck portion, it contacts the inner surface of the neck portion. Here, since an enamel layer having a predetermined thickness is present on the inner surface of the neck portion, when the bulb spacer is mounted at a proper position it scratches the enamel layer, which provides for a path for discharge. Also, the particles of the scratched enamel layer float between the focus electrode and the final accelerating electrode, resulting in a discharge between electrodes.
To solve the above problems, it is an object of the present invention to provide a CRT which can definitely reduce convergence drift with a metal coating layer on the inner surface of a neck portion.
Accordingly, to achieve the above object, there is provided a cathode ray tube having reduced convergence drift including a panel having a phosphor layer on its inner surface, a funnel sealed in the panel and having an inner graphite layer and an outer graphite layer on inner and outer surfaces, respectively, an electron gun in a neck portion of the funnel and consisting of a cathode, a control electrode, a screen electrode, a plurality of focusing electrodes, a final accelerating electrode and a shield cup, and a metal coating layer electrically connected with the inner graphite layer on the inner surface of a neck portion and having the surface resistivity of 107xcexa9/xe2x96xa1 or less.
Here, the metal coating layer is preferably formed on the inner surface of the neck portion higher than the top surface of the focusing electrodes.
Also, the metal coating layer is preferably electrically connected with the final accelerating electrode via the shield cup.
Also, the metal coating layer may be selectively formed on the inner surface of the neck portion adjacent to side electron beam apertures for red and blue electron beams.
Further, the metal coating layer is preferably a metal thin film having either iron or chrome as a main component. Also, the metal coating layer may be a metal thin film having both iron and chrome as main components.